Cells comprising the rat caput epididymidis were separated using centrifugal elutriation. Enriched populations contained an average of 72.1 plus or minus 3.7% principal cells, 86.9 plus or minus 2% basal cells or 70.9 plus or minus 6.5% fibroblasts. Cell viability in all fractions was 88%. Glycerolphosphorylcholine (GPC) and sialic acid levels were significantly greater (P less than or equal to .01) in principal cells than in basal cells, fibroblasts or caput and cauda spermatozoa. No significant differences in GPA or sialic acid levels were observed among the latter cell types. Polyacrylamide gel electrophoresis (PAGE) was used to investigate the protein composition of isolated rat caput epididymal cells, washed caput, corpus and cauda sperm, cauda tubular fluid and rete testis fluid. Some proteins are apparently secreted by the epididymis and become associated with luminal sperm and/or fluid. Other proteins may be removed from rete testis fluid as it enters the epididymis. Studies are currently in progress to develop techniques to separate cells comprising the corpus epididymidis. Separated corpus somatic cells and sperm will be analyzed for GPC, sialic acid, glycosidases and proteins. These studies should elucidate the regional functions of the epididymis and suggest contraceptive approches to altering its functions.